The present invention relates generally to electrochemical cells, and particularly to zinc-chlorine batteries combined to form a battery plant system.
Due to the increasing demand for electricity and the decreasing availability (and increasing cost) of distillate oil and natural gas, the need has arisen for an alternate method of supplying peak demand electricity. Presently, the electricity generated for peak demand is supplied from diesel engines and combustion turbines, which are fired by distillate oil and natural gas. One such method is the use of secondary energy storage batteries to store electricity generated from utility baseload facilities during the night or off-peak hours, and discharging these batteries during the hours of peak demand. Secondary energy storage batteries currently being considered for this application include lead-acid, lithium-iron sulfide, sodium sulfur, sodium-chloride, and zinc-chlorine batteries. In order to be utilized in this application, these batteries would necessarily have to be scaled up to battery plants capable of delivering electrical energy on the order of 100 mega-watt hours in a single discharge. This scale up would generally be achieved by combining large numbers of cells into module-type units, and interconnecting a suitable number of these modules.
One of the primary concerns in such a scale up, is the reliability of the battery plant. This reliability may generally be characterized as a function of the number of battery module failures. Since these modules would usually be connected electrically in series to form battery strings, the failure of a single module will affect the operation of the entire string. If the failure is such that the battery string must be disconnected from the electrical current flow in the battery plant, this has the effect of the failure of all of the battery modules in the string.
The present invention provides a novel battery plant system and method of redirecting the electrical current around a failed module, so that only the failed module is removed and the reliability of the battery plant is maximized. Particularly, the battery plant according to the present invention employs a bypass switch for each of the battery modules in the battery plant. The bypass switch contains a normally open main contact across the power terminals of the battery module and a plurality of normally closed auxiliary contacts for controlling the supply of reactants electrochemically transformed in the cells of the battery module. Thus, open the determination of a failure condition, the bypass switch is energized to close the main contact and concomitantly open the auxiliary contacts. With the supply of reactants to the cells turned off, the electrical current through the battery will rapidly decrease, and the current through the battery string will be redirected through the bypass switch main contact.
The present invention further provides for the employment of battery string isolation switches which need not have to break d.c. current when the entire battery string must be disconnected in response to a failure condition. This is achieved by first turning off the supply of reactants to the entire battery string by energizing at least one auxiliary switch. Again, within a short time after the supply of reactants is turned off, the current through the battery string will substantially decrease, and allow the isolation switch for the string being disconnected to open under essentially no-load conditions.
Although the description is directed to zinc-chlorine batteries, the present invention may also be utilized in conjunction with suitable electrochemical cells which store the reactants remote from the site of reaction, and which can tolerate high short-circuit currents. Other features and advantages of the invention will become apparent in view of the drawings and the following detailed description of the preferred embodiments.